The present invention relates generally to anchoring apparatus and more specifically to clamp apparatus for securing an inverted boat to the roof of a motor vehicle. Motor vehicles can be provided with roof carriers for the purpose of carrying large objects that cannot adequately fit inside the vehicle. These carriers have been widely used for transporting small water vessels such as fishing boats. When secured in the ordinary manner, the bow and stern of the boat are tied to the front and rear of the motor vehicle, respectively. However, it is still possible that a strong crosswind or air turbulence created by a passing truck can force the boat sideways off the roof of the motor vehicle. The present invention solves this problem by providing clamp apparatus that connects the sides of the boat to the motor vehicle.